It is well known in the electronic arts to manufacture semiconductor junctions for a variety of purposes. For example, diodes are formed using a single junction, and transistors, thyristors, and other devices are formed with multiple junctions. Silicon is the most commonly used semiconductor material but other semiconductor materials are also well known.
Diodes or rectifiers are the simplest types of semiconductor devices formed using a single PN junction. Many different types of rectifiers are known. For example, some are constructed in epitaxial layers formed on semiconductor substrates using planar technology. While very small devices may be formed in this fashion, such techniques are not particularly desirable for large area rectifiers because of the high material cost associated with the epitaxial wafers, the higher process costs associated with the planar technology, and the more limited reverse voltage capability of the resulting rectifier.
Another commonly used technology for constructing power rectifiers is to form the rectifier in a "mesa" structure. Sometimes these are formed in epitaxial material and sometimes directly in the original starting substrate. The use of the mesa type structure provides improved control of the fringing electric fields at the edges of the PN junction where it meets the surface so that the reverse leakage current is reduced and reverse voltage breakdown increased.
Mesa diodes constructed on epitaxial material however are expensive because of the high cost of the starting material. While the material cost for non-epitaxial mesa structures is lower, in order to achieve reasonable forward voltage drops, the starting substrate material must be extremely thin, or must be etched to reduce its thickness during processing. Cutting the mesas reduces the mechanical strength of the wafer. Such wafers are more likely to break during processing, reducing overall manufacturing yield and increasing cost.
Accordingly, there continues to be a need for improved structures and methods for forming semiconductor junctions used for rectifiers and other devices, which overcome one or more limitations of the prior art.